Embryology

All major organ systems are formed between the fourth and eighth weeks of development (Table 1–1). This is called the period of organogenesis. It is during this time that the embryo is most susceptible to factors that interfere with development.

The cardiovascular system, including the heart, blood vessels, and blood cells, originates from the mesodermal germ layer.¹⁸ Cardiovascular morphogenesis is controlled by mechanisms that are common to all developmental processes: cell growth, cell migration, cell death, differentiation, and adhesion.

The heart initially consists of paired tubular structures that by the twenty-second day of development (the embryo is approximately 2.5 to 3 mm in length) form a single, slightly bent heart tube (Fig. 1–1). This heart tube consists of an inner endocardial tube and a surrounding myoepicardial mantle. At this stage, the heart tube connects with the developing arch system and with the vitelline and umbilical veins.¹⁹

The second stage of cardiac development begins with the formation of the atrioventriculobulbar loop. The cephalic portion of the heart tube bends ventrally and to the right, whereas the caudal atrial portion begins to bend in a dorsocranial direction and to the left, thus forming a loop (Fig. 1–2).¹⁸

As this heart loop continues to bend, a common atrium is formed and enters the pericardial cavity, carrying along the right and left segments of the sinus venosus. From here the atrioventricular canal forms, which connects the common atrium to the early embryonic ventricles.

It is at this time (approximately 28 days) that contractions are thought to begin in the ventriculobulbar portion of the heart, and the heart beat is initiated.²⁰ Circulation occurs from the sinus venosus into the right atrium, into the left atrium, and then into the atrioventricular canal and the ventricles.

Stage three in the development of the heart consists of absorption of the bulbus cordis and sinus venosus. At this stage, the atrioventriculobulbar loop begins to untwist and the cardiac septa develop, forming a four-chambered heart.¹⁹

Formation of the septa within the heart results from the development of endocardial cushion tissue in the atrioventricular canal and the truncoconal region. This occurs between the twenty-seventh and thirty-seventh days of development, when the embryo is 4 to 14 mm in length (Fig. 1–3).

In the atrium, the septum primum—a sickle-shaped crest descending from the roof of the atrium—does not completely divide the atrium in two, but leaves an open ostium primum for communication between the two chambers. When the ostium primum is obliterated owing to fusion of the septum primum with the endocardial cushions, the ostium secundum forms within the septum primum.

Last, a septum secundum is formed, but an interatrial opening, the foramen ovale, remains until birth when pressure in the left atrium increases, causing the two septa to press against each other and close this communication.²⁰

Septum formation within the atrioventricular canal occurs when two large endocardial cushions fuse, resulting in a right (tricuspid) and left (mitral) atrioventricular orifice (Fig. 1–4). This usually occurs by day 33 of development.¹⁸

The interventricular septum is formed by the end of the seventh week of development (Fig. 1–5). It results from the dilation of the two primitive ventricles (right and left conus swellings), which causes the medial walls to become apposed and fuse together. This forms the muscular portion of the interventricular septum. Formation of the membranous portion follows.

During the eighth week of development, the truncus swellings or cushions of the primitive heart grow and twist around each other to form the aorticopulmonary septum (Fig. 1–6). This septum divides the truncus arteriosus into an aortic channel and a pulmonary channel.

The cushions of the conus cordis develop simultaneously. These conus cushions unite with the aorticopulmonary septum. After this fusion occurs, the septum divides the conus into an anterolateral portion (the right ventricular outflow tract) and a posteromedial portion (the left ventricular outflow tract).

Next, the opening that remained between the two ventricles closes as a result of the conus septum fusing with tissue from the inferior endocardial cushions along the top of the muscular interventricular septum (see Fig. 1–5). This becomes the membranous part of the interventricular septum.^18,19

Between 5 and 7 weeks of development, the semilunar valves (aortic and pulmonic valves) are formed.

Aortic Arches

During the fourth and fifth weeks of development, six pairs of arteries arising from the most distal portion of the truncus arteriosus are formed (Table 1–2). They are known as aortic arches (Fig. 1–7). These arches form communications between the aortic sac and the two dorsal aortas.

TABLE 1–2

Timing of Aortic Arch Formation

Timing	Event
Days 19–20	First pair of aortic arches form
Days 20–23	Second pair of aortic arches form
Days 24–25	Third pair of aortic arches form
	First and second pairs of aortic arches disappear
	Third pair of aortic arches becomes the internal carotid arteries
Days 26–30	Fourth pair of aortic arches form
	Fourth left arch becomes the definitive aorta
	Fourth right arch becomes the right subclavian artery
	Fifth pair of aortic arches never fully develop
Day 30	Sixth pair of aortic arches form
	Sixth right arch becomes right pulmonary artery
	Sixth left arch becomes left pulmonary artery (ductus arteriosus)

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